Abstract
We investigate the photoinduced dynamics of perylene diimide dyads based on a donor-spacer-acceptor motif with polyyne spacers of varying length by pump-probe spectroscopy, time resolved fluorescence, chemical variation and quantum chemistry. While the dyads with pyridine based polyyne spacers undergo energy transfer with near-unity quantum efficiency, in the dyads with phenyl based polyyne spacers the energy transfer efficiency drops below 50%. This suggests the presence of a competing electron transfer process from the spacer to the energy donor as the excitation sink. Transient absorption spectra, however, reveal that the spacer actually mediates the energy transfer dynamics. The ground state bleach features of the polyyne spacers appear due to the electron transfer decay with the same time constant present in the rise of the ground state bleach and stimulated emission of the perylene energy acceptor. Although the electron transfer process initially quenches the fluorescence of the donor it does not inhibit energy transfer to the perylene energy acceptor. The transient signatures reveal that electron and energy transfer processes are sequential and indicate that the donor-spacer electron transfer state itself is responsible for the energy transfer. Through the introduction of a Dexter blocker unit into the spacer we can clearly exclude any through bond Dexter-type energy transfer. Ab initio calculations on the donor-spacer and the donor-spacer-acceptor systems reveal the existence of a bright charge transfer state that is close in energy to the locally excited state of the acceptor. Multipole-multipole interactions between the bright charge transfer state and the acceptor state enable the energy transfer. We term this mechanism coupled hole-transfer FRET. These dyads represent a first example that shows how electron transfer can be connected to energy transfer for use in novel photovoltaic and optoelectronic devices.
Highlights
The probability for Forster resonant energy transfer (FRET) decreases with the sixth power of the donor–acceptor distance and has a much larger range of action
Due to the approximations done to get to this multipole–multipole description, there are many instances where the theory breaks down and mechanisms that go beyond Forster theory are required to explain the resonant energy transfer.[10,11,12,13,14,15,16]
We have recently shown that in a perylene diimide dyad based on a donor–spacer–acceptor motif (D–(Me4-Ph)–A in Chart 1) rapid FRET from the donor to the
Summary
The probability for FRET decreases with the sixth power of the donor–acceptor distance and has a much larger range of action. The Forster radii (the donor–acceptor distance at which the rate of energy transfer equals that of fluorescence of the excited donor) lie in the order of 30 Å. We have recently shown that in a perylene diimide dyad based on a donor–spacer–acceptor motif (D–(Me4-Ph)–A in Chart 1) rapid FRET from the donor to the. The electron transfer times between perylene diimides and electron rich phenyl spacers are in the order of up to 10 ps at most.[21] With energy transfer times ranging from 10 to 50 ps this would lead to FRET efficiencies below 10%, far less than observed. We investigate the photoinduced dynamics of these dyads by pump–probe spectroscopy, time resolved fluorescence and quantum chemical calculations in order to elucidate the steps comprising the full energy transfer mechanism
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